A. Field of the Invention
This invention relates to the field of art of automatic positioning control systems having solenoid operated valve assemblies.
B. Prior Art
Prior positioning control valve systems have produced undesirable fluid shock waves following the opening or closing of the valve. When a valve closes, for example, the total amount of fluid is passed through a valve orifice and the fluid acts on a mass downstream causing an abrupt displacement of the mass. Specifically, the mass may be said to be instantly accelerated by the moving fluid which acceleration results in a shock force. Such shock is applied to the mass from the moving fluid which may produce damage to the mass or components.
This problem positioning control has been particularly important in the contour following mobile equipment field such as for example, in agricultural combines which have very large springy tires. A combine carries a heavy header which may weigh, for example, 2,000 lbs. When the header is hydraulically moved, any shock waves which are produced are transmitted first to the chassis of the combine and then to the tires and results in extremely objectionable oscillations.
A further problem in combine operation has been that after finishing a row, it has been important that the header be rapidly raised and taken out of the row. In the rough terrain of the field, the combine must then be turned, realigned with the row and then the header rapidly lowered back into the field to begin a new cut. As the new cut is being made, the header must be adjusted at high speeds within fractions of an inch in order to maintain proper cutting height. The header must operate within a "dead band" the limits of which are required to be kept at a minimum as for example from one to one and a half inches. Within that narrow dead band, the header must operate and avoid overshoot and undershoot at normal cutting speeds. Thus, a single positioning control hydraulic system must provide rapid raising of a very heavy header load at the end of a row and after the cut has been started, the hydraulic system must then accurately and carefully maintain a very narrow dead band at a desired ideal cutting height with minimized overshoot and undershoot.
Accordingly, an object of the present invention is to provide for a heavy load a narrow dead band with minimal overshoot and undershoot.
Another object of the present invention is to also minimize the shock on the heavy load by means of the parabolic contour of the valve and the sizing of the orifices for providing substantially linear flow rate change when the valve moves.